As you sit at your desk and chew your morning muffin, consider the complicated interlocking structure of your jaw. How did you manage to evolve such a thing? What you’re eating through is essentially a badly deformed but useful gill, and we’ll tell you how it got to be that way.

Worm World

The world used to be a simple place. It was warmer, with a lot of water, and in that water were simple beings that would suck in water one place and blow it out another. This is how they got everything that they needed. Structures in their mouth-holes would sift the incoming debris for food, and, as the water was excreted through their gills, they strained the oxygen from it. There wasn’t any other mechanism of eating. The planet was populated, almost exclusively, by swimming earthworms.

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If you spend all day sucking in water, it’s important to have a reliable way to spit that water back out again immediately. If your water-spitter is the only thing giving you oxygen, it’s even more important that it not collapse or get stuck together. The advantages of supporting the gill slits with something hard and structured became obvious. The swimming earthworms started keeping their gills open with boney structures, but these early skeletons looked absolutely nothing like what we currently picture when we think of fish skeletons. They would be missing out on structures like a recognizable cranium, or well-developed places for eyes. All they had were arches going from one side of the fish to the other supporting those proto-gills.

The Paired Fish

This fish, called Metaspriggina, doesn’t look like a game-changer, but it’s the first step towards modernity. Metaspriggina, which emerged 505 million years ago, has all sort of wild innovations: It has, internally, the beginnings of a cranium. It has a post-anal tail. (You’re welcome, monkeys.) But what is most remarkable about it is it has pairs. Those little snail-stalks sticking out front are a pair of eyes. Behind them are a pair of nostrils. And behind them, inside the fish, are “an array of bipartite bars.” Instead of continuous slits supported by a single structure, this fish had pairs of gills supported by pairs of bones.

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These pairs of bones could do all kinds of things. Each one could, slowly, over time, evolve into a different structure. Some could move back and fuse with the cranium. Some could become supports for different modern facial structures. And the front pair could migrate forward, gain a little more mobility, and make the mouth into more than just a tube for sucking in water.

The Vampire Tank

As much as they’ve looked, scientists haven’t yet really found a half-jaw. There are no fish yet on the record with primitive, semi-moving jaws or gill arches that are close enough to the mouth to manipulate it but too far to be considered actual jaws. What there are, however, are 400-million-year-old placodermi.

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These fish didn’t have scales to protect themselves. Instead, they had slender, vulnerable tails and massive arm-plated bodies. These fish were tanks, with articulated plates running up and down the front of their bodies and huge, heavy skulls. Their jaws were not particularly advanced. For some time, scientists didn’t believe they had modern jaw bones, and instead had the undersea equivalent of beaks. Then a fossil in China showed that they had a lower jaw bone, making them (or rather their ancestor) the first step in what eventually became the annoying loud gum-chewer on the bus every morning.

It’s worth noting that what they didn’t have was teeth. This is why scientists often believed they had more of a beak than a face. The bony plates at the front of their face were incredibly hard and rubbed against each other—which we believe caused them to be self-sharpening. Each time this fish chased down its terrified, jaw-less prey and snapped it up, it was making its own mouth-equipment sharper. In fact, many placodermi fossils show what look like well-developed fangs.